The present invention relates to a resistive bridge interface circuit; and more particularly, to a resistive bridge interface circuit with reduced power consumption.
As shown in FIG. 1, a conventional resistive bridge interface circuit 10 for use with a strain gage, for example, is a conventional Wheatstone bridge configuration. The Wheatstone bridge interface circuit 10 includes two arms 12, 14 coupled in parallel between the input power terminals VDD 16 and Ground 18. The first arm 12 includes resistors R120 and R222 connected in a series and the second arm 14 includes resistors R324 and R426 connected in series. A first voltage output terminal V128 is positioned between the two resistors R120 and R222 in the first arm 12, and a second voltage output terminal V230 is positioned in series between the two resistors R324 and R426 of the second arm 14. When used as an interface circuit of a strain gage, the resistors R120, R222, R324 and R426 are variable resistors disposed upon a diaphragm, where the resistance of such resistors vary with the tensile strain experienced by the resistors. These resistor all have substantially the same initial resistance value. Resistors R120 and R426 both vary in a first direction in response to the strain experienced thereby; and resistors R222 and R324 vary in an opposite direction in response to the strain experienced thereby. The magnitude of variance experienced by each resistors in response to the strain experienced by each resistors is substantially the same. Accordingly, if the diaphragm experiences strain, resistors R120 and R426 may experience a decrease in resistance while resistors R222 and R324 may experience a substantially identical increase in resistance. This in turn creates an imbalance across the bridge 10 such that when a voltage is applied at the input terminals VDD 16 and Ground 18, an output voltage Vout occurs across output terminals V128 and V230, which is related to the movement of the diaphragm relative to the pressure being sensed. Assuming that the resistors have the same initial resistance, R, this output voltage Vout can be expressed mathematically in accordance with the following equation       V    out    =                    Δ        ⁢                  xe2x80x83                ⁢        R            R        ·          VDD      .      
If the power driving this interface circuit 10 is a DC power supply, the power consumption for this circuit is VDD2/R. If the power supply is a square wave or sign wave signal, then a demodulation circuit is applied to obtain a DC output signal, and power consumption is only xc2xd of the power consumption as compared to the DC power supply.
The present invention is directed to a resistive bridge interface circuit; and more particularly, to a resistive bridge interface circuit with relatively lower power consumption.
A first aspect of the present invention is directed to a resistive bridge interface circuit that includes: (a) a pair of input power terminals; and (b) a resistive bridge circuit that includes a pair of parallel resistive branches coupled between the pair of input power terminals; (c) where each resistive branch of the resistive bridge interface circuit includes a voltage output terminal and a switch incorporated in series with the resistive branch; and (d) where each switch is activated at a duty cycle.
In a more detailed embodiment of this first aspect of the present invention, at least one of the resistive branches includes a variable resistor incorporated in series therewith.
In an alternate detailed embodiment of this first aspect of the present invention, each resistive branch includes a variable resistors incorporated in series therewith, where such variable resistors vary in response to an environmental condition in opposite directions with respect to each other. In the more detailed embodiment, the environmental condition is strain experienced by the variable resistors.
In another alternate embodiment of the first aspect of the present invention, the resistive bridge interface circuit further includes a differentiator operatively coupled to the voltage output terminals. In a more detailed embodiment, the differentiator is a switched-capacitor differential amplifier circuit.
It is a second aspect of the present invention to provide a resistive interface circuit that includes: (a) first and second input power terminals; (b) first and second parallel branches coupled in series between the first and second input power terminals; (c) where the first branch includes a first voltage output terminal, a first current source coupled in series between the first input power terminal and the first voltage output terminal, a first resistor coupled in series between the first voltage output terminal and the second input power terminal, and a switch positioned in series between the first input power terminal and the first voltage output terminal; and (d) where the second branch includes a second voltage output terminal, a second resistor coupled in series between the first input power terminal and the second voltage output terminal, a second current source coupled in series between the second voltage output terminal in the second input power terminal, and a switch positioned in series between the second voltage output terminal and the second input power terminal. In a more detailed embodiment, the first and second switches are configured to be activated simultaneously. In a further detail of embodiment, the first and second switches are configured to be activated simultaneously at a relatively low duty cycle. In yet a further detailed embodiment, the first and second resistors are variable resistors. In yet a further detailed embodiment, the first and second resistors are adapted to vary proportionately with respect to each other in response to an environmental condition. In yet a further detailed embodiment, the first and second resistors vary in opposite directions with respect to each other. In yet a further detailed embodiment, the environmental condition varying the first and second resistors is strain experienced by the first and second resistors. In yet a first detailed embodiment, the first and second resistors have substantially the same initial resistance values. In yet a further detailed embodiment, the current source comprises a third resistor coupled in series between the first input power terminal and the first voltage output terminal, the second current source comprises a fourth resistor coupled in series between the second voltage output terminal and the second input power terminal, and the third and fourth resistors have substantially the same resistance values. In yet a further detailed embodiment, the third and fourth resistors are variable resistors with substantially the same initial resistance value as the first and second resistors and are adapted to vary and opposite directions with respect to each other in response to strain experienced by the third and fourth resistors. In yet a further detailed embodiment, the relatively low duty cycle is approximately a 15% duty cycle to approximately a 50% duty cycle.
In an alternate detailed embodiment to the second aspect of the present invention, the first and second resistors are variable resistors with substantially the same initial resistance value and vary proportionately in opposite directions with respect to each other in response to an environmental condition. In a further detailed embodiment, the first and second current sources respectively comprise third and fourth variable resistors with substantially the same initial resistance value of the first and second resistors and which vary proportionately in opposite directions with respect to each other in response to the environmental condition, and the first and third variable resistors vary proportionately in opposite directions with respect to each other in response to the environmental condition, and, likewise, the second and fourth variable resistors vary proportionately in opposite directions with respect to each other in response to the environmental condition. In yet a further detailed embodiment, the environmental condition is strain experienced by the first, second, third and fourth resistors.
In yet another alternate detailed embodiment of the second aspect of the present invention, the first and second switches are operatively coupled to, and activated by a single timing circuit.
In yet a further alternate detailed embodiment to the second aspect of the present invention, the first switch is coupled in series between the first input power terminal and the first current source, and the second switch is coupled in series between the second input power terminal and the second current source.
In yet a further detailed embodiment to the second aspect of the present invention, the first switch is coupled in series between the first current source in the first voltage output terminal and the second switch is coupled in series between the second current source and the second voltage output terminal.
In yet another alternate detailed embodiment of the second aspect of the present invention, the resistive bridge interface circuit further includes a differentiator operatively coupled to the first and second voltage output terminals. In a more detailed embodiment, the differentiator is a switched-capacitor differential amplifier.